Water in an above-ground swimming pool is recirculated by a pump for filtration. It is sometimes necessary to undertake maintenance on the filter or the pump. If the pump and filter are lower than the pool water level, it is advisable to close a valve in each of the draw pipe and return pipe prior to removal of the filter or the pump, to prevent unwanted drainage of the pool. Similarly, if the filter and pump are located higher than the pool water level, the valves should be closed to prevent drainage of the filter and pump, as many pumps can be damaged if they are run unprimed.
To prevent such damage, close-off valves are commonly provided in the draw and return pipes connecting the pump and filter to the pool. It is important that these valves are not closed during operation of the pump, as the pump or the pipes themselves could be damaged by excessive back-pressure. However, above-ground swimming pools are often used by children, and inquisitive or naughty children might be tempted to play with the valve taps.